The invention relates to digitizing apparatus, especially digitizing apparatus including a tablet with embedded X and Y conductors and a pen, cursor, or other transducer having a conductive stylus or tip that transmits electric signals that are capactively coupled to the embedded conductors in order to produce signal levels in the embedded conductors indicative of the coordinates of the stylus; the invention also relates to a computer input device of a type commonly referred to as a "mouse", and more particularly to a low cost, low power digitizing system including a cordless pen/mouse or a cordless keyboard.
Electronic digitizers which include a horizontal "tablet" having a grid of spaced X and Y conductors embedded beneath a digitizing surface of the tablet, and also a "pen" with a writing tip or stylus that can be lowered onto the digitizing surface, are well known. Some devices of this type are based upon electrostatic, or capacitive, coupling between the conductive stylus or cursor and the digitizing surface. Other systems rely on inductive, or electromagnetic, coupling between the digitizing surface and the pen or cursor also have been utilized. The two basically different design approaches have both been used, to meet different objectives. Systems which use inductive coupling between the pen and the digitizing surface typically must use ten to one hundred times more power than is the case if capacitive coupling approaches are used, and are not suitable for systems in which very low power dissipation is a major objective. U.S. Pat. Nos. 3,444,465, 3,767,858, 3,851,097, 4,034,155, 4,158,747, 4,227,044, 4,492,819, 4,451,698, and 4,491,688 perhaps are generally illustrative of the state of the art.
Presently known systems typically utilize a high level of power, and are characterized by large amplitude pen or grid signals which couple the digitizing surface to the cursor in order to achieve adequate reliability in the presence of relatively high electrical noise levels. Although a cordless cursor device, such as the one disclosed in U.S. Pat. No. 4,451,698, has been proposed, no cordless digitizer stylus structure has been utilized in a low power, low cost digitizing system, especially one of the type utilizing electrostatic coupling between the stylus and the digitizing surface. This is probably the case because of the unacceptably high weight of a self-contained battery power supply, and also because of the absence of a good electrical ground. A good electrical ground ordinarily would be necessary in a system that utilizes electrostatic coupling. The device disclosed in above-mentioned U.S. Pat. No. 4,451,698 is based upon inductive coupling between the cordless cursor and the digitizing surface, and uses much higher signal levels, and dissipates much more power than would be dissipated in a system using electrostatic or capacitive coupling between the pen and the digitizer surface.
There is an unmet need for a low cost, high resolution digitizing device utilizing a stylus which is cordless, and thereby more convenient to use, than one having an electrical cable connecting it to the digitizing tablet, and also having good noise immunity to normally present levels of ambient noise, and also having the capability to function as a mouse as well as a digitizing device.